Today's power plants utilizing geothermal heat are placed on the surface of the earth and utilize 2 small holes for water downwards and steam upwards, but they reached never 20 km deepness. The drilling of small holes has big disadvantages; the drill head must be attached to the drilling rig on the surface and the torque must be transmitted over a very long distance.
The bore head can only be repaired by taking it out all the way. There are also not yet shaft drilling machines for vertical drilling to 10-20 km.
The temperatures are not high enough everywhere for a good efficiency of the carnot process. The temperature rises about 33° C. every kilometer down, somewhere even less. The greater temperature and pressure the greater is the power and efficiency of the turbines. Ergo we have to go deeper in the earth.
Furthermore the turbines on the surface of the earth can't utilize the energy of the water falling in great depths.